1. Field of the Invention
The present invention relates to a liquid electrophotographic printer for performing a printing operation using an ink as a developer which is composed of a power toner and a liquid carrier, i.e. solvent, and more particularly, to a squeezing apparatus of a liquid electrophotographic printer capable of removing the developer left after the developing operation and the carrier of the developer.
2. Description of the Prior Art
Generally, an electrophotographic printer such as, for example, a laser printer, obtains a desired image by the processes of: forming an electrostatic latent image on a photosensitive medium such as a photosensitive drum or photosensitive belt; developing the electrostatic latent image with a toner of a certain color; and transferring the developed electrostatic latent image onto a printed medium. Electrophotographic printers are mainly divided into a wet type and a dry type, depending on the type of toner the printer uses. The wet type electrophotographic printer uses a developer which is composed of a volatile liquid carrier and a toner. The wet type electrophotographic printer is in demand, since it has a higher print quality than the dry type which uses a powder toner, and cannot be damaged by harmful toner dust such as is used in the dry type printer.
FIG. 1 schematically shows the main portions of a wet type electrophotographic printer, which is described below.
As shown in FIG. 1, the wet type electrophotographic printer includes a photosensitive belt 1 disposed on an endless track, and first 2, second 3, and third 4 rollers for rotatably moving the photosensitive belt 1 on its destined path. Here, the third roller 4 is driven by a driving motor (not shown) to rotate the photosensitive belt 1, and the second roller 3 is in the form of a steering roller for preventing the photosensitive belt 1 from skewing by adjusting the tension.
Further, around the photosensitive belt 1, a charging unit 10, exposure unit (not shown), developer unit 20, drying unit 30, and transfer/fixing unit 40 are disposed. Here, the charging unit 10 uniformly charges the photosensitive belt 1, and the exposure unit (not shown) forms an electrostatic latent image on the photosensitive belt 1 by projecting a laser beam onto the photosensitive belt 1 according to an image signal. Further, the developer unit 20 develops the electrostatic latent image on the photosensitive belt 1 by attaching the developer having a certain color toner on the image formative area of the photosensitive belt 1. For a color printer, there are a plurality of exposure units (not shown) for mixing the respective colors, and a plurality of developer units 20 for holding respective colors of toner.
The image formed on the photosensitive belt 1 by the toner and the developer unit 20 is transferred to the printed matter by a transfer roller 41 which is disposed parallel to the first roller 2, while having the photosensitive belt 1 therebetween. The desired image is obtained on the printed matter as the printed matter is fed between the transfer roller 41 and a fixing roller 42, which is parallel from the transfer roller 41 and spaced at a certain interval.
As shown in FIGS. 2 and 3, the developer unit 20 is disposed adjacent to the lower portion of the photosensitive belt 1, and includes a casing 50, developing device 60, and squeezing device 70.
The developing device 60 includes a developing roller 61 which is driven-rotated to selectively come into tight contact with the photosensitive belt 1, developing backup roller 62, and developer jetting nozzle 63 for feeding the developer between the photosensitive belt 1 and the developing roller 61.
The squeezing device 70 performs the first removal of a residual developer left after the developing operation of the developing device 60, and also a liquid carrier from the developer. Here, the liquid carrier has to be removed from the developer during the developing operation, since an excess of liquid carrier on the image formed by the toner hinders the image transference of the transfer/fixing unit 40. The carrier which is not removed by the developer unit 20 is removed to a certain degree by the dry unit 30 which is appropriate for image transference.
The squeezing device 70 is described in greater detail with reference to FIG. 4.
As shown in FIG. 4, the squeezing device 70 includes a squeezing roller 71, squeezing backup roller 72, and a squeezing brush 73.
The squeezing roller 71 and the squeezing backup roller 72 are pressed into tight contact with the photosensitive belt 1 by a certain pressure (approximately of 20 kg/f) by a pressing section which is not shown, to thus squeeze out unnecessary liquid carrier from the developer applied on the photosensitive belt 1 by passive-rotational movement thereof.
Further, during the squeezing operation, the squeezing brush 73 removes the carrier and/or ink toner attached on the squeezing roller 71 by its rotational movement in tight contact with the outer circumference of the squeezing roller 71. The squeezing brush 73 is rotated by the driving force transmitted from a developer roll driving section which is not shown in the drawings.
Further, albeit not shown, the squeezing device 70 includes a driving section for reverse-rotating the squeezing roller 71 during a drip line removing mode, and a squeezing blade 74 (See FIGS. 2 and 3) for removing the ink toner attached on the reverse-rotating squeezing roller 71 in tight contact with the outer circumference of the squeezing roller 71, which is reverse-rotated by the driving section.
As shown in FIG. 2, the squeezing device 70 constructed as defined above squeezes out and removes the carrier from the developer applied on the surface of the photosensitive belt 1 by its reverse-rotational movement while being pressed into tight contact with the surface of the photosensitive belt 1 at a certain pressure. Here, the squeezing brush 73 is in contact with the squeezing roller 71, and accordingly, the squeezing brush 73 removes the carrier and ink toner attached on the squeezing roller 71 while being rotated faster than the squeezing roller 71 by the driving force transmitted from the developer roller driving section.
After completion of the printing mode, a drip line removing mode is performed in which the ink toner unnecessarily attached on the photosensitive belt 1 is removed. In this situation, as shown in FIG. 3, the squeezing roller 71 is pressed with less pressure than in the printing mode, and the squeezing blade 74 is rotated by the driving section in a reverse direction from the rotation direction of the printing operation in contact with the squeezing roller 71, to thus attract unnecessary ink toner from the photosensitive belt 1 thereto during its reverse-rotation. The ink attached on the reverse-rotating squeezing roller 71 is then removed by the squeezing blade 74 which is in contact with the outer circumference of the squeezing roller 71. In such a situation, the developer roller 61 is spaced from the photosensitive belt 1 at a certain interval, and the squeezing brush 73 is spaced from the squeezing roller 71 at a certain interval.
In the squeezing apparatus of the liquid electrophotographic printer operating as above, it is very important to rotate the squeezing roller 71, which is passive-rotated in contact with the photosensitive belt 1, at the same speed as the travel speed of the photosensitive belt 1 without causing a slip occurrence. In the event of slippage due to different relative speeds of the photosensitive belt 1 and the squeezing roller 71, the image formed on the photosensitive belt 1 is smeared and blurred, causing print quality deterioration. Accordingly, the slippage must be prevented during the passive-rotation of the squeezing roller 71.
In the conventional squeezing apparatus, however, since the squeezing brush 73 in contact with the squeezing roller 71, which is passive-driven in contact with the photosensitive belt 1 is rotated faster than the squeezing roller 71, torque is transmitted to the squeezing roller 71 from the squeezing brush 73, and the squeezing roller 71 is therefore rotated faster than the photosensitive belt 1. Accordingly, slipping occurs, i.e., the squeezing roller slides on the photosensitive belt 1. As a result, the image is inevitably blurred.